1. Field of the Invention
The present invention relates to a defibrillation system of the type employing a defibrillator housing to which at least two defibrillation electrodes are connected, with at least one of these electrodes being intended for intracardiac placement.
2. Description of the Prior Art
Defibrillation systems of the type generally described above are known wherein the intracardiac electrode includes a flexible electrode cable with at least one elongate, electrically insulated conductor and at least one defibrillation surface disposed at a distal end of the electrode cable for delivering defibrillation pulses to the heart. Such known systems include control means for determining when defibrillation therapy, in the form of one or more defibrillation pulses, is to be administered via the intracardiac electrode and the other electrodes.
Such a defibrillation system is disclosed in U.S. Pat. No. 4,662,377. This system includes two defibrillation electrodes, one of which is subcutaneous and the other of which is intracardiac Both electrodes are connected to the defibrillator housing. The electrode head of the intracardiac electrode is a helical conductor, having a diameter roughly corresponding to the external diameter of the electrode cable. This conductor extends along a substantial portion of the distal end of the electrode. A disadvantage of such a defibrillation electrode is that it has a defibrillation area which is relatively small, compared to the magnitude of the current which is applied to this area during the delivery of a defibrillation pulse. This can result in damage to the heart due to burning, since the defibrillation surface often presses against the heart wall. As a result of the size, location and concentration of the defibrillation area at one site in the heart, optimum defibrillation is not achieved.
A defibrillation electrode is disclosed in U.S. Pat. No. 5,010,894 intended for intracardiac placement. The electrode head of this known defibrillation electrode is formed by a plurality of outwardly-projecting, pre-curved flexible conductors, which serve as defibrillation surfaces. The proximal ends of the conductors are anchored in adjacent manner in a common connection device at the same time as their distal ends are adjacently anchored to a second common connection device. Before an electrode cable is introduced into a heart via a vein, the electrode head is stretched using a stylet so that the conductors are brought close to each other, thereby giving the electrode head a diameter which is only slightly larger than the diameter of the electrode cable. After the electrode head has been advanced into the heart, the stylet can be withdrawn, thereby permitting the conductors to expand laterally so as to resiliently press against the surrounding wall along a substantial portion of their length. Current applied through this defibrillation electrode can be evenly distributed to these conductors, which jointly form a relatively large defibrillation area. This can prevent burn damage to the surrounding heart wall. Optimum defibrillation can be achieved, because the conductors can be evenly distributed inside the heart. Because the electrode head is permanently lodged in the heart, at least the conductors can be made of a material having excellent properties for permitting the conductors to follow the movements of the heart for a very long period of time without damage to the conductors themselves, or to the heart. Such a relatively large electrode head, however, can impede the flow of blood in the heart.